FIELD OF THE INVENTION
The present invention relates generally to laser beam machining and, more specifically, to laser beam machining of non-circular cross-section apertures.
A gas turbine engine includes a compressor for compressing air, which is mixed with fuel and ignited for generating hot combustion gases in a combustor with energy being extracted from the gases in a turbine, disposed downstream of the combustor for powering the compressor and providing output power. Various components bound the hot combustion gases and, therefore, are typically cooled during operation for obtaining a useful life thereof. One conventional cooling arrangement includes film cooling holes which are typically inclined at an acute angle through the component for receiving a portion of the compressed air on one side thereof which passes through the holes to form a film of cooling air along the opposite side of the component which provides effective film cooling of the component during operation. Film cooling holes are typically found in combustion liners, turbine nozzle vanes and blades, turbine shrouds, and various shields requiring effective cooling. One particular type of a film cooling hole is a diffuser hole or a shaped hole. Typically, a diffuser hole or slot is formed with an exit area on the surface of the component to be cooled and the exit area is larger than its entrance area. Often a circular or otherwise shaped constant area feed hole is formed from the other surface of the component to the entrance area of the diffuser portion of the aperture. From the apertures, the cooling air is discharged along the cooled surface for creating a continuous cooling air film within the boundary for cooling the component.
Conventional processes exist for forming through holes in gas turbine engine components such as disclosed wherein holes may be drilled using an industrial laser or electrical discharge machining (EDM). In conventional laser drilling, a suitably powered laser beam is maintained at a desired location as the beam vaporizes metal until the through hole is completed. The laser, such as a ND:YAG laser, is typically operated at a suitable pulse rate with each pulse vaporizing a portion of the metal until the entire hole is completed.
However, methods to form these so-called blind apertures in the component for the improved film cooling diffuser apertures or slots, creates significant problems and costs. There is also a desire to form these noncircular cross-sectional diffuser portions of the aperture in an inexpensive manner while still being able to provide effective film cooling. The sidewalls and bottom of the slot should preferably be relatively smooth for aerodynamic reasons for efficient film cooling operation. The walls and bottom should also be sufficiently smooth to avoid undesirable stress concentrations for limiting maximum stress in the component during operation for providing a long life.
In the typical YAG pulse laser, each pulse has an amplitude and a finite duration, each typically having a plus or minus 5% variation in value. By integrating over time, the power of each pulse, the amount of energy in each pulse, may be determined which can have about a plus or minus 20% variation based on the worst case combination of the plus and minus 5% variations on pulse amplitude and duration. This substantial energy variation means that the amount of metal vaporized per pulse varies significantly from pulse to pulse, with the corresponding aperture being formed by consecutive pulses, varying significantly in configuration during the process. This significant pulse energy variation is typically not a major concern for drilling or laser cutting, since the objective is to form a through hole or cut, with the configuration of the in-process hole or cut being immaterial.
However, this substantial energy variation is quite significant for attempting to form blind non-circular holes or slots which do not pass completely through the metal component. Accordingly, attempting to use a laser in conventional practice to drill a blind hole and, then, continue the process for forming an elongated non-circular hole or blind slot, will result in a diffuser portion of the aperture having a substantial variation in width, depth and surface contour. The resulting jagged contour diffuser would be undesirable for aerodynamic and strength reasons. The jagged contour decreases the ability to form a substantially smooth and uniform cooling air film and the jagged bottom of the slot could undesirably decrease the effective strength of the base portion of the component below the slot bottom. Turbine film cooled components are typically relatively thin in overall thickness, which requires the accurate placement and depth of the blind aperture and diffuser hole or slot therein. If the remaining base material below the diffuser is too thin, the components may have undesirably low strength either reducing its useful life or requiring rejection of the component during the manufacturing process.
Alternatively, conventional EDM machining may be used for accurately forming the blind diffuser, followed in turn by forming the required through holes in the slot, again using conventional EDM machining or conventional laser drilling. These two processes would typically be performed in two separate and distinct steps utilizing refixturing the component in the same or different machines to form the differently configured blind slot and through holes in the component. Refixturing presents the additional problem of maintaining accurate alignment between the blind diffuser and the feed holes therein. Accordingly, the resulting manufacturing process would be relatively complex and costly since a typical component such as a turbine blade, turbine vane, or combustion liner has a substantial number of film cooling through holes, with the corresponding large number of blind apertures associated therewith and many blades and vanes are manufactured with these film cooling apertures.